The metal-air battery is a battery which uses oxygen existing in the air as a positive active material and uses a metal as a negative active material. The metal-air battery is characterized in that energy density (electric energy capable of discharging per weight) is high, and downsizing and weight saving are easy since the positive active material does not need to be stored within a battery. The battery currently receives attention as a battery having energy density exceeding a lithium ion battery having high energy density.
Such a metal-air battery is composed of a positive electrode, a negative electrode, an electrolyte, a separator, and an oxygen permeating membrane formed on the positive electrode. A carbon material such as graphite or acetylene black is used for the positive electrode. However, a heretofore used carbon material has a problem that only a capacity which is far from a theoretical capacity of the metal-air battery can be realized.
On the other hand, Patent Document 1 proposes a positive electrode predominantly composed of a carbonaceous substance in which a pore volume of fine pores having a diameter of 1 nm or more is 1.0 ml/g or more. Then, Patent Document 1 describes that the larger the pore volume of fine pores having a diameter of 1 nm or more is, the more the diffusibility of ions and oxygen is enhanced.
Further, Patent Document 2 states a carbon material including fine pores containing mesopores and micropores having a pore size smaller than that of the mesopore in which a carbonaceous wall constituting a shell of the mesopore forms a three-dimensional network structure, the micropore is formed at the carbonaceous wall, the mesopore is an open air hole, and an open air hole portion consecutively forms a combined pore. It is described that since the carbonaceous wall forms a three-dimensional network structure, the diffusibility of lithium ions and oxygen is improved, thus performance is improved.